1. Field of the Invention
This invention relates to a display control device, and more particularly, to a display control device for a display unit having display elements which use, for example, a ferroelectric liquid crystal as an operating medium for updating display, and which can maintain a display state updated by the application of an electric field, or the like.
2. Description of the Related Art
In general, a display unit, serving as an information display means having a function of visually displaying information is connected to an information processing system or the like. CRTs (cathode-ray tubes) have been widely used as such display units. A display control device for a CRT connected to an information processing system is illustrated in FIG. 12.
In FIG. 12, an address bus driver 1, a control bus driver 2 and a data bus driver 3 are connected to system bus 4 for connecting respective units constituting an information processing system with signals. A video memory 5 stores display data transmitted via the data bus driver 3. A driver 6 transmits data from a display control device to a CRT 7.
The video memory 5 comprises a dual port DRAM (dynamic random access memory), in which display data are directly written. Display data written in the video memory 5 are sequentially read by a CRTC (CRT controller) 8, and are displayed on the CRT 7.
That is, in writing display data, a CPU (not shown) of the information processing system accesses an address of the video memory 5 corresponding to an display area on the CRT 7. First, a request signal for the access is supplied to a memory controller 9 via the control bus driver 2, where the signal is subjected to arbitration with a data transfer request signal or a refreshing request signal provided from the CRTC 8. In accordance with this operation, in the memory access by the CPU, an address selection signal is supplied from the memory controller 9 to an address selector 10, and an access address for data writing from the CPU is supplied to the video memory 5 via the address driver 1 and the address selector 10. In accordance with this operation, a DRAM control signal from the memory controller 9 and the display data via the data bus driver 3 are supplied to the video memory 5. The display data are thereby written in the video memory 5.
Display on the CRT 7 is executed in the following manner. That is, the CRTC 8 supplies the driver 6 with a synchronizing signal. In synchronization with the synchronizing signal, the CRTC 8 supplies the memory controller 9 with a data transfer request signal, and also supplies the address selector 10 with a data transfer address.
First, the data transfer request signal is subjected to arbitration by the memory controller 9. In accordance with this operation, an address selection signal is supplied from the memory controller 9 to the address selector 10. The data transfer address from the CRTC 8 is then supplied to the video memory 5 via the address selector 10. A DRAM control signal is supplied from the memory controller 9 to the video memory 5, and a data transfer cycle is thereby executed. The data transfer cycle indicates the transfer of data in units of a line (corresponding to a raster on the picture surface of the CRT 7) stored in the video memory 5 to a shift register within the video memory 5. Data from one line to several lines can be transferred to the shift register by one data transfer cycle.
The display data transferred to the shift register are sequentially read from the shift register by a serial port control signal supplied from the CRTC 8 to the video memory 5, and are output to and displayed on the CRT 7. The reading of the display data from the video memory 5 and the display of the read data are performed by a so-called entire-surface refreshing operation, wherein the reading and display are performed from the upper portion to the lower portion of a display area in units of a line, and from the left end to the right end on a line according to a predetermined order.
As described above, in the display control of the CRT 7, the writing operation of the CPU for the video memory 5 and the reading/displaying operation of display data from the video memory 5 by the CRT controller 8 are independently performed.
In the case of the above-described display control device for the CRT, since the writing operation of display data for the video memory 5 for changing display information and the like, and the reading/displaying operation of display data from the video memory B are independently performed, the device has the advantage that display timing and the like need not be considered in a program of the information processing system, and it is therefore possible to write desired display data with an arbitrary timing.
On the other hand, since the CRT particularly requires a certain length in the direction of the thickness of the display picture surface, its volume becomes large as a whole, and it is therefore difficult to make the entire display unit small. As a result, the freedom in the use of an information processing system using a CRT as a display unit, that is, the freedom in the place of installation, portability and the like is reduced.
In order to solve the above-described problems, a liquid crystal display (hereinafter termed an LCD) may be used. That is, by adopting an LCD, the entire display unit may be provided having a small size (particularly having a thin size). As one type of LCDs, there is a display unit using a ferroelectric liquid crystal (hereinafter termed an FLC) cell (hereinafter termed an FLCD). One of the features of the FLCD is that the FLC cell can maintain its display state after the application of an electric field. Hence, in driving the FLCD, there is an allowance in time for a period of continuously refreshing driving of the display picture surface, contrary to the CRT and other liquid crystal displays. Furthermore, independently of the continuous refreshing driving, it becomes possible to perform partial rewriting driving to update the display state of only a portion to be changed on the display picture surface. Accordingly, such an FLCD can provide a display unit having a larger picture surface than other liquid crystal displays.
The FLCD has a sufficiently thin liquid crystal cell. Molecules of slender FLCs within the cell are oriented in a first stable state or a second stable state in accordance with the direction of an applied electric field, and maintain their orientation state even after switching off the electric field. The FLCD has a storage capability due to such bistability of the FLC molecules. The details of such FLC and FLCD are described in, for example, Japanese Patent Application No. 62-76357 (1987).
However, when using the FLCD having the above-described advantages, as a display unit of an information processing system using the same display control as that of the CRT, the FLCD is in some cases incapable of following such a change in display information that a display must be instantaneously rewritten, for example, in the case of a cursor, a character input, scrolling and the like, since a time required for a display updating operation of the FLC is relatively long.
In a configuration utilizing the capability of partial rewriting which is one of the features of the FLCD, in order to perform the partial rewriting processing, for example, the information processing system side provides information for identifying the partial rewriting processing. In order to realize partial rewriting driving on the above-described display picture surface, however, a control program of the information processing system must be largely changed.